I've been in Japan for almost a quarter of my life, which happened a lot faster than you might think. In those seven years, for the sake of communication with non-native speakers, I've had to make a lot of small compromises in my English, both spoken and written. I've found myself foregoing a lot of natural sentence structures and segues, and breaking my sentences down into their simplest forms. Grammar becomes fixed in the old subject-verb-object pattern, because God help you if you decide to use the passive voice. If you think native speakers have a hard time with past participles, imagine what it's like for all those poor non-natives. Sit-sat-sat, but hit-hit-hit? Drink-drank-drunk, sink-sank-sunk, but...think-thought-thought? Madness. Don't even start on how we use pronouns. To Japanese listeners, English speakers are probably some of the most sadistic conversationalists, outdone only by the French.
To communicate with Japanese people, you have to make verbal sacrifices, one at a time, until your vocabulary dries up and your English hardens into the formal, structured, and less nuanced style of someone with almost no personality. While your words once flowed like a stream, they now are placed carefully, like bricks in a wall, perfectly squared and leveled, ready for the next thought to be laid on top. Of course, this is wonderful for listeners, but it's terrible for anyone who loves finding that one special word that captures any of the humor, irony, or wit that might be bouncing around in your head.
At the same time, it makes reading "current" English all the more painful. Quickly checking Twitter feeds or Facebook posts will lead you into an avalanche of acronyms, euphemisms and portmanteaus. If you leave the Internet for more than a week, you might find yourself in dire need of a Google search bar and (lord help us) a dedicated window for urbandictionary.com.
So, how can native speakers abroad maintain their natural English writing and speaking levels? The same way you'd maintain any muscle or skill. You have to exercise, and make sure never to fall into a routine. This means you're going to have to read from many different sources, from poorly written tweets to literary classics, newspapers to blogs. Then, write a blog of your own. You'll need to be listening to more diverse music, and watching movies and TV shows you might have thought were a waste of time. Yes, reality TV has value to those of us living abroad. Scripted TV is becoming more and more about witty, snappy dialogue, but no one actually speaks that way, unless they want to be known as an endless source of one-liners. If you've ever heard a bunch of Americans abroad finally getting together after weeks of isolation, you know that their conversations aren't conversations at all, just a chance to let loose all the cool phrases they came up with when they had no one else to talk to but themselves. Sad, really. But, reality TV, while the situations may be scripted, the speaking style is not. You won't be making any public speeches with that kind of talk, but at least you won't be so surprised when you go home and realize that your friends get by talking about nothing in particular...and are OK with that. And finally, if you're lucky enough to be in a time zone that sees sunlight at the same time as your home country, call people. As often as you can. Your brain needs it. If you're like me, and everyone you care about back home is asleep when you're awake...you're probably still looking for a way around it.
Just remember that your native language is just like any other muscle. You gotta work out. You can't rest on the laurels of that one great speech you made at a party, or that ice-cold reply you shot back at a jerk on a message board. Switch it up, push yourself, and enjoy every form of your mother language. It'll remind you just how lucky you are to have one.